Synthesis and use of heterogeneous polymer gels

ABSTRACT

Disclosed is a heterogeneous polymer gel comprising at least two gel networks. One embodiment of the present invention concerns a heterogeneous polymer gel comprising a first gel network comprising an environmentally-stable gel and a second gel network comprising an environmentally-unstable gel wherein the first gel network interpenetrates the second gel network. The heterogeneous polymer gel exhibits controlled changes in volume in response to environmental changes in condition, such as of temperature or of chemical composition.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional patent application of U.S.patent application Ser. No. 08/768,163, filed on Dec. 13, 1996, now U.S.Pat. No. 5,976,648 which claims the benefit of priority to provisionalU.S. patent application Ser. No 60/008,644, filed on Dec. 14, 1995,abandoned both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heterogeneous polymer gel comprisingat least two gel networks. The heterogeneous polymer gel exhibitscontrolled changes in volume in response to external environmentalchanges.

2. Description of the Related Art

Relatively uniform or homogeneous polymer gels have attracted muchinterest because their volume can be changed by several orders ofmagnitude in response to the change of an external factor such assolvent, temperature, electric field, or light. Uses of such homogeneouspolymer gels include such applications as artificial muscles,drug-delivery devices, chemical valves and actuators, and magneticresonance monitoring agents.

It is desirable to develop and produce new polymer gels that exhibitdesired reversible characteristics in response to external environmentalchanges.

SUMMARY OF THE INVENTION

In one aspect, the present invention concerns a heterogeneous polymergel comprising at least two gel networks. The heterogeneous polymer gelis characterized in that it exhibits desired changes in volume inresponse to external environmental changes.

One embodiment of the present invention concerns a heterogeneous polymergel comprising a first gel network comprising an environmentally-stablegel and a second gel network comprising an environmentally-unstable gelwherein the first gel network interpenetrates the second gel network.

One embodiment of the present invention concerns a heterogeneous polymergel comprising a first gel layer comprising an environmentally-stablegel and a second gel layer comprising an environmentally-unstable gelwherein the first gel layer is attached to the second gel layer.

In another aspect, the present invention concerns a method of using aheterogeneous polymer gel comprising at least two gel networks.

In one embodiment of the present invention, a method of using aheterogeneous polymer gel, comprising an environmentally-stable gel andan environmentally-unstable gel, comprises heating the heterogeneous gelto a temperature effective to result in a desired change in volume inthe environmentally-unstable gel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention concerns a class of materials based on the spatialmodulation of the chemical nature of gels. The modulation is achieved byallowing only part of one gel network to interpenetrate with another gelnetwork. The resultant gels have an internally heterogeneous ormodulated structure.

The heterogeneous polymer gel of the present invention generallycomprises a first polymer gel network comprising anenvironmentally-stable polymer gel and a second polymer gel networkcomprising an environmentally-unstable polymer gel wherein the firstpolymer gel network interpenetrates the second polymer gel network.

As used herein, “an environmentally-stable polymer gel” is intended torepresent a polymer gel that remains substantially inert or unchanged asthe environment surrounding the polymer gel undergoes a change inconditions, such as in physical or chemical conditions. In particular,it is desired that the environmentally-stable polymer gel does notexhibit a substantial change in volume as the conditions of theenvironment surrounding the environmentally-stable polymer gel undergoesa change.

As used herein, “an environmentally-unstable polymer gel” is intended torepresent a polymer gel that exhibits a substantial change in itsphysical characteristics as the environment surrounding the polymer gelundergoes a change in conditions, such as in physical or chemicalconditions. In particular, it is desired that theenvironmentally-unstable polymer gel exhibits a substantial change involume as the conditions of the environment surrounding theenvironmentally-unstable polymer gel undergoes a change.

Polymer gels that have been found to be useful in the present inventioninclude N-isopropylacrylamide gels and polyacrylamide gels.

As used herein, the “environment” for a polymer gel is intended torepresent the physical surroundings of the polymer gel. Generally, theenvironment of a polymer gel will be a liquid or a gas surrounding thepolymer gel. For example, the environment of a polymer gel may be anaqueous solution or air.

In order to cause a substantial change in the physical characteristicsof an environmentally-unstable polymer gel, it is generally necessarythat the environment surrounding the environmentally-unstable polymergel undergoes a change in conditions. As will be appreciated by thoseskilled in the art, the change in environmental conditions necessary tocause a substantial change in the physical characteristics of anenvironmentally-unstable polymer gel will generally be dependent on thespecific environmentally-unstable polymer gel being used in a particularheterogeneous polymer gel. Such a change in conditions can, for example,be a change in the temperature, the pH, or the chemical composition ofthe environment.

In the present invention, it is generally desired that the environmentis substantially uniform and/or that the change in conditions be appliedto the environment in a substantially uniform manner. As such, it isgenerally not necessary to induce a gradient of an environmentalproperty, such as temperature or electric field, to the environment inorder to achieve the desired substantial change in the physicalcharacteristics of an environmentally-unstable polymer gel.

In one embodiment of the present invention, an ionicN-isopropylacrylamide gel, prepared with sodium acrylate, shrinkssubstantially in volume when in an aqueous solution at temperatureshigher than about 37° C. In contrast, a polyacrylamide gel does notundergo a substantial change in volume when in an aqueous solution attemperatures higher than about 37° C. As such, a heterogeneous polymergel may be prepared comprising polyacrylamide gel as theenvironmentally-stable polymer gel and an ionic N-isopropylacrylamidegel as the environmentally-unstable polymer gel, wherein the environmentis an aqueous solution, and the change in conditions for the environmentwill be a change in temperature from less than about 37° C. to greaterthan about 37° C.

In another embodiment of the present invention, a polyacrylamide gelshrinks substantially in volume when in a water/acetone solution thatcomprises at least about 34 weight percent acetone. In contrast, anN-isopropylacrylamide gel does not undergo a substantial change involume when in a water/acetone solution that comprises at least about 34weight percent acetone. As such, a heterogeneous polymer gel may beprepared comprising an ionic N-isopropylacrylamide gel as theenvironmentally-stable polymer gel and a polyacrylamide gel as theenvironmentally-unstable polymer gel, wherein the environment is awater/acetone solution, and the change in conditions for the environmentwill be a change in acetone concentration from less than about 34 weightpercent to greater than about 34 weight percent.

As such, as will be appreciated by one skilled in the art, a polymer gelthat is used as an environmentally-stable polymer gel in oneenvironment, or under a particular change in conditions for thatenvironment, may alternatively be used as an environmentally-unstablepolymer gel in another environment, or under a different change inconditions for the original environment.

As also will be appreciated by one skilled in the art, it may bedesirable to selectively choose the environmentally-stable polymer geland the environmentally-unstable polymer gel that are being used so asto maximize the difference in properties between the two gels relevantto the change in conditions that are intended to be made to anenvironment. For example, if the temperature of the environment is to bechanged so as to induce a change in the volume of theenvironmentally-unstable polymer gel, it may be desirable to maximizethe difference in the thermal expansion coefficients of the two gels. Inparticular, an ionic N-isopropylacrylamide gel may shrink to half of itsoriginal size per degree Celsius near the transition point of about 37°C. In contrast, a polyacrylamide gel essentially has no change in volumenear this temperature. As such, the difference in thermal expansioncoefficients for these two gels can be as high as about 0.5 per degreeof Celsius. In contrast, the difference of expansion coefficients for atypical bimetallic strip of brass and steel is about 7×10⁻⁶ per degreeof Celsius. Thus, bigels can generally be made to be much more sensitiveto environmental changes than can bimetals.

The form of a prepared heterogeneous polymer gel will depend to a largeextent on the use for which it is intended. One suitable form is a bigelstrip comprising two gel networks or layers in contact with each otherwherein one of the gel networks interpenetrates the other gel network.When placed into an environment, and then subjecting the environment toan effective change in conditions, such a bigel strip will typicallychange in shape, bending for example, from an essentially straight shapeto almost a circle.

Another suitable form is a bigel composite comprising a gel matrixprepared from a first polymer gel in contact with, at least at twoseparate locations, gel networks of a second polymer gel. When placedinto an environment, and then subjecting the environment to an effectivechange in conditions, such a bigel composite will typically change inshape, bending, for example, from an essentially straight shape to apentagon shape and then to a quadrangle shape.

Another suitable form is a bigel composite comprising a gel matrix inthe shape of a cylinder prepared from a first polymer gel that includesat least two gel network strips of a second polymer gel that are locatedin different locations around the circumference of the cylinder. Whenplaced into an environment, and then subjecting the environment to aneffective change in conditions, such a bigel composite will typicallychange in shape with the ends of the cylinder bending relatively openand then relatively closed. Such a change in shape may allow the bigelcomposite to grasp or release an object as the environmental conditionschange.

The amount of bending a bigel composite exhibits as the environmentalconditions change may be quantified by the bending angle, theta (θ),defined to be the angle between the two tangents of a gel arc at twoends. The strain (s) induced in a gel is then equal to:

s=(θd)/L

wherein “d” represents the thickness of the gel and “L” represents thelength of the gel. This equation should be valid even for large bending.Using this equation, it was found that a strain of up to about 50percent could be produced in a bigel composite.

The properties of heterogeneous polymer gels of this invention wouldenable them to function as gel display devices, switches, valves, andother similar applications.

EXAMPLES Example 1

A bigel strip was prepared by first making a N-isopropylacrylamide gelslab. Two glass slides with about a 1.0 millimeter gap between them wereimmersed in about 100 milliliters of an aqueous solution of about 690millimoles of N-isopropylacrylamide, about 8.6 millimoles ofmethylene-bis-acrylamide, and about 8 millimoles of sodium acrylate. Thepolymerization of the solution was initiated by the addition of about240 microliters of tetramethylethylenediamine and about 40 milligrams ofammonium persulfate. About 20 percent of the water in theN-isopropylacrylamide gel was evaporated for a better subsequentinterpenetration process. The resulting product was anN-isopropylacrylamide gel slab having a thickness of about 1.2millimeters.

In the second step, the N-isopropylacrylamide gel slab prepared in thefirst step was placed between, but in contact with, one of two glassslides with about a 2 to 3 millimeter gap between them, such that thedistance between the two glass slides was larger than the thickness ofthe N-isopropylacrylamide gel slab. The glass slides andN-isopropylacrylamide gel slab were immersed in an acrylamide gelsolution prepared from about 700 millimoles of acrylamide and about 8.6millimoles of methylene-bisa-crylamide. The acrylamide gel solution wasallowed to diffuse into the N-isopropylacrylamide gel slab for about onehour before polymerization of the acrylamide was initiated, thus,ensuring the formation of a N-isopropylacrylamide/polyacrylamideinterpenetrating network. The end product was a bigel slab about 2 to 3millimeters thick with a layered network structure having apolyacrylamide gel network about 0.8 to about 1.8 millimeters thick andan N-isopropylacrylamide gel network interpenetrated by a polyacrylamidegel network about 1.2 millimeters thick.

The heterogeneous bigel slab was immersed in water at about 22° C. Thebigel slab swelled and bent slightly toward the polyacrylamide gelnetwork side. The temperature of the water was gradually increased toabout 37.8° C. The N-isopropylacrylamide gel network was observed toshrink substantially, whereas the polyacrylamide gel network wasgenerally insensitive to the temperature change. As a result, the bigelslab bent into an arc towards the N-isopropylacrylamide gel networkside. The transition between the straight and arc positions was found tobe reversible as the temperature is varied between about 22° C. andabout 37.8° C. The variation in the degree of bending in nine sequentialstraight-bending-straight cycles was less than about 3 percent.

The heterogeneous bigel slab was immersed in a water-acetone solutioncomprising about 20 percent acetone. The bigel slab swelled but remainedrelatively straight. The heterogeneous bigel slab was then immersed in awater-acetone solution comprising about 45 percent acetone. Thepolyacrylamide gel network was observed to shrink substantially, whereasthe N-isopropylacrylamide gel network was generally insensitive to theacetone change. As a result, the bigel slab bent into an arc towards thepolyacrylamide gel network side.

Example 2

A gel with a more complex modulated structure was prepared in a mannersimilar to that used for the bigel slab of Example 1. A polyacrylamidegel network was modulated with a N-isopropylacrylamide gel network atfour locations. Four N-isopropylacrylamide gel pieces were placed on aglass plate with adjacent gel pieces separated by about 12 millimeters.An acrylamide gel solution was then allowed to diffuse into theN-isopropylacrylamide gel pieces for about one hour beforepolymerization of the acrylamide was initiated, thus, ensuring theformation of N-isopropylacrylamide/polyacrylamide interpenetratingnetworks. The end product was a bigel slab about 2 to 3 millimetersthick with a layered network structure having a polyacrylamide gelnetwork about 0.8 to about 1.8 millimeters thick and fourN-isopropylacrylamide gel networks interpenetrated by a polyacrylamidegel network about 1.2 millimeters thick.

The heterogeneous bigel slab was immersed in water at about 22° C. Thebigel slab swelled and was essentially straight. The temperature of thewater was gradually increased to about 39° C. Each of theN-isopropylacrylamide gel networks were observed to begin bending to anangle of about 70 degrees, whereas the polyacrylamide gel network wasgenerally insensitive to the temperature change. As a result, the bigelslab bent into a pentagon-like shape. The temperature of the water wasgradually increased to about 41° C. Each of the N-isopropylacrylamidegel networks were observed to further bend to an angle of about 90degrees, whereas the polyacrylamide gel network was still generallyinsensitive to the temperature change. As a result, the bigel slab bentinto a quadrangle-like shape. The transition between the differentpositions was found to be reversible as the temperature was variedbetween the different temperatures.

Example 3

Two bigel strips were prepared according to the procedure of Example 1.The end products were bigel slabs about 3 to 4 millimeters thick with alayered network structure having a polyacrylamide gel network about 2.1millimeters thick and an N-isopropylacrylamide gel networkinterpenetrated by a polyacrylamide gel network about 1.5 millimetersthick.

A gel “hand” was prepared using the two bigel strips that were tiedtogether at one end with a spacer. The N-isopropylacrylamide gelnetworks of each bigel strip face each other. The structure was immersedinto water at about 22° C. The bigel slabs swelled and bent slightlytoward the polyacrylamide gel network sides so that the end of thestructure was relatively open. The temperature of the water wasgradually increased to about 35° C. The N-isopropylacrylamide gelnetworks were observed to shrink substantially, whereas thepolyacrylamide gel networks were generally insensitive to thetemperature change. As a result, the bigel slabs bent towards each otherso that the end of the structure was relatively closed. As the structureclosed, the structure enclosed or grasped a small object that wasfloating in the water. As the temperature of the water was reduced toabout 22° C., the bigel slabs returned to being relatively straight sothat the end of the structure opened and released the small object.

What is claimed is:
 1. A method of using a heterogeneous polymer gel toproduce reversible characteristics in response to external environmentalchanges, wherein the heterogeneous polymer gel comprises a first gelnetwork comprising an environmentally-stable gel and a second gelnetwork comprising an environmentally-unstable gel wherein the first gelnetwork interpenetrates the second gel network, the method comprisingplacing the heterogeneous polymer gel into a first environment whereinthe environmentally-unstable gel exhibits a first volume and thenplacing the heterogeneous polymer gel into a second environment whereinthe environmentally-unstable gel exhibits a second volume.
 2. The methodof claim 1 wherein the second volume is less than the first volume. 3.The method of claim 1 wherein the second volume is greater than thefirst volume.
 4. The method of claim 1 wherein theenvironmentally-stable gel comprises polyacrylamide, theenvironmentally-unstable gel comprises N-isopropylacrylamide, the firstenvironment is an aqueous solution having a temperature that is lessthan about 37° C., and the second environment is an aqueous solutionhaving a temperature that is greater than about 37° C.
 5. The method ofclaim 1 wherein the environmentally-stable gel comprisesN-isopropylacrylamide, the environmentally-unstable gel comprisespolyacrylamide, the first environment is a first solution comprisingwater and acetone wherein the first solution comprises less than about34 weight percent of acetone, and the second environment is a secondsolution comprising water and acetone wherein the second solutioncomprises greater than about 34 weight percent of acetone.
 6. The methodof claim 1 wherein the first environment has a pH, the secondenvironment has a pH, and the second environment has a pH that isgreater than the pH of the first environment.
 7. The method of claim 1wherein the first environment has a pH, the second environment has a pH,and the second environment has a pH that is less than the pH of thefirst environment.
 8. The method of claim 1 wherein the firstenvironment has a temperature, the second environment has a temperature,and the second environment has a temperature that is greater than thetemperature of the first environment.
 9. The method of claim 1 whereinthe first environment has a temperature, the second environment has atemperature, and the second environment has a temperature that is lessthan the temperature of the first environment.
 10. A method of using aheterogeneous polymer gel to produce reversible characteristics inresponse to external environmental changes, wherein the heterogeneouspolymer gel comprises a first gel network comprising anenvironmentally-stable gel and a second gel network comprising anenvironmentally-unstable gel wherein the first gel networkinterpenetrates the second gel network, the method comprising placingthe heterogeneous polymer gel into a first environment wherein theheterogeneous polymer gel exhibits a first bending angle and thenplacing the heterogeneous polymer gel into a second environment whereinthe heterogeneous polymer gel exhibits a second bending angle.
 11. Themethod of claim 10 wherein the second bending angle is greater than thefirst bending angle.
 12. The method of claim 10 wherein the secondbending angle is less than the first bending angle.
 13. The method ofclaim 10 wherein the environmentally-stable gel comprisespolyacrylamide, the environmentally-unstable gel comprisesN-isopropylacrylamide, the first environment is an aqueous solutionhaving a temperature that is less than about 37° C., and the secondenvironment is an aqueous solution having a temperature that is greaterthan about 37° C.
 14. The method of claim 10 wherein theenvironmentally-stable gel comprises N-isopropylacrylamide, theenvironmentally-unstable gel comprises polyacrylamide, the firstenvironment is a first solution comprising water and acetone wherein thefirst solution comprises less than about 34 weight percent of acetone,and the second environment is a second solution comprising water andacetone wherein the second solution comprises greater than about 34weight percent of acetone.
 15. The method of claim 10 wherein the firstenvironment has a pH, the second environment has a pH, and the secondenvironment has a pH that is greater than the pH of the firstenvironment.
 16. The method of claim 10 wherein the first environmenthas a pH, the second environment has a pH, and the second environmenthas a pH that is less than the pH of the first environment.
 17. Themethod of claim 10 wherein the first environment has a temperature, thesecond environment has a temperature, and the second environment has atemperature that is greater than the temperature of the firstenvironment.
 18. The method of claim 10 wherein the first environmenthas a temperature, the second environment has a temperature, and thesecond environment has a temperature that is less than the temperatureof the first environment.